We know how nasty the Zenins are. How toxic, traumatizing and inhumane they're to any member who isn't born with CE and awaken a CT. And those are reasons why Maki and Toji left that house. But Maki seems to have left with fewer emotional and mental scars than Toji imo.

And I think for all the above, Toji didn't annihilate his clan as Maki did. They were the only family he knew, and he wanted to belong somewhere. So he spared them, and went to work to prove his worth. He didn't spare them on a whim.

I think that's one reason for Toji to remarry and have Mrs. Fushiguro take care of his son (one more reason is to throw away the Zenin name and free himself and Megumi from them), but Mrs. Fushiguro leaves a while later too.

While Toji was separated from his son he meets Naobito and makes a deal for Megumi's sake. He must have known that living with Mrs. Fushiguro wasn't the best choice for his son, and he promised his late wife to take care of him.

Toji probably never wanted to sell his son, but he thought that to be for the best. Also acting like an ah as to not get too emotionally attached to Megumi. Hoping maybe if Megumi thinks badly about him then he won't live a lonly life longing for his father forever.